Assembly process development of 50um fine pitch wire bonded devices

With the demand on cost competitiveness, continuous reduction of silicon area has a significant impact on circuit design, wafer fabrication, testing and package assembly process. Shrinking chip size and increasing I/O counts require finer bond pad pitch and bond pad size for circuitry layout. Wafer probing becomes quite challenging within the finer area for wire bonding process. Also wire sweep/short after transfer molding process becomes quite critical as a smaller wire diameter has to be applied for the finer bond pad pitch. This paper covers wire bonding and molding process development for 50 mum fine pitch wire bond plastic ball grid array package (PBGA). Since a large percentage of the total bonding area was damaged during probe testing, the adhesive strength of the ball bond was reduced. Bond lift issue has been observed after destructive wire pull test. A comparative study of different probe mark size using 0.9 mil wire diameter was presented and analysis on bonded units concludes that the critical damage threshold was around 20% of bonded area. A possible solution with irregular elliptical ball bonding was proposed to improve adhesive strength of ball bonds. On the other hand, reduced wire diameter has decreased the bending strength of bonded wires significantly, and thus causing serious wire sweep after transfer molding. Experiments were conducted to evaluate wire length with 0.9 mil diameter gold wire. The results show that the wire threshold length is about 4.1 mm. A novel solution was applied using Non-Sweep method. The wire length could be extended to 5.0 mm within 1.0% wire sweep. Reliability tests conducted showed that all the units passed 1000 temperature cycles (-55degreesC similar to 125degreesC) with JEDEC moisture preconditioning at level 2a (60degreesC/60% relative humidity for 120 hours) and 3 times reflow (peak temperature at 220-225degreesC). It is believed that this solution can efficiently overcome the risk of wire shorting and improve the yield of ultra fine pitch wire bonds in high volume production.